In 1991, David G. Bailey, PhD, and colleagues found that grapefruit juice increased blood concentrations of the blood pressure drug Plendil to possibly dangerous levels. Grapefruit juice, they later learned, slows down a key liver enzyme that clears Plendil — and about 40 other drugs — from the body.

Now Bailey reports that grapefruit, orange, and apple juices decrease the absorption of several important medications:

"This is just the tip of the iceberg. I'm sure we'll find more and more drugs that are affected this way," Bailey says in a news release.

Bailey revealed the new findings in a report to the 236th annual meeting of the American Chemical Society.

A substance in grapefruit juice called naringin seems to be the culprit. The compound apparently blocks OATP1A2, a transporter molecule in the gut, which carries some drugs from the small intestine into the blood. Orange juice contains hesperidin, a naringin-like substance. The culprit in apple juice remains unidentified.

"The concern is loss of benefit of medications essential for the treatment of serious medical conditions," Bailey says.

In their studies, Bailey and colleagues had healthy volunteers take fexofenadine with either a glass of grapefruit juice, a glass of water mixed with naringin, or pure water. Taking the drug with grapefruit juice or the naringin mixture halved the amount of drug that reached the bloodstream.

People should take their pills only with water, advises Bailey, a professor of clinical pharmacology at the University of Western Ontario, London, Canada. He suggests that people taking medications should check with their doctor or pharmacist before taking medications with fruit juices or whole fruits.